The present invention relates to a food refrigeration system and method having two or more cooling stages in which food is refrigerated by transferring heat from the food to cooling water and then to a cryogen causing the cryogen to boil off into a cryogenic vapor. The present invention also relates to a method and apparatus for cooling articles that has particular application to the aforementioned food refrigeration system and method in which the cooling potential of the cryogenic vapor is recovered in the cooling water by drawing the cryogen vapor into a mixture with the cooling water through use of an eductor.
The prior art has provided food refrigeration systems for refrigerating articles of food by transferring heat from the articles of food in sequential cooling stages. In such refrigeration systems, a first of the cooling stages can comprise a hydrocooler and a second of the cooling stages can comprise a cryogenic freezer, such as a liquid nitrogen immersion freezer or a post cooling tunnel used in conjunction with a liquid nitrogen immersion freezer or a carbon dioxide spiral belt freezer. A third cooling stage can be formed by a mechanical freezer. The hydrocooler, by provision of a waterbath or water shower arrangement, pre-chills the food with cooling water, either ground water supplied by a ground water well or tap water supplied by a municipal water supply or brine. In the cryogenic freezer, food is chilled to a temperature above freezing, crust frozen, or frozen throughout by a bath or spray of cryogen. When a cryogenic freezer is used in crust freezing food, the mechanical freezer is provided to complete the freezing of the food by transferring heat from the food to a recirculating refrigerant such as ammonia.
The number and types of cooling stages used in a particular food refrigeration system relate to the degree of refrigeration required and the particular food involved. For example, a food refrigeration system used in freezing fruit may consist of, in order, a hydrocooler, a liquid nitrogen immersion freezer and post cooling tunnel, and a mechanical freezer. In such a food refrigeration system, the hydrocooler is used to pre-chill the fruit, the liquid nitrogen immersion freezer and post cooling tunnel are used to crust freeze the fruit and the mechanical freezer is used to freeze the fruit throughout. The initial crust freezing of the fruit prevents the cellular damage to the fruit that would otherwise occur if the fruit were solely frozen throughout in a mechanical freezer. Another food refrigeration system may be formed by a hydrocooler and a liquid nitrogen immersion freezer without a post cooling tunnel; or a hydrocooler and a carbon dioxide spiral belt freezer. Such a refrigeration system could be used to freeze vegetables throughout without first crust freezing the vegetables. A liquid nitrogen immersion freezer may be used alone as a chiller to prevent articles of food, which in one stage of their processing are hot and sticky, from sticking to conveyor belts employed in moving the food through a food processing system. In accordance with the above discussion, it is to be noted that the term, "refrigeration" as used herein encompasses freezing, crust freezing and chilling.
In any of the food refrigeration system, discussed above, the cryogen in the cryogenic freezer is continually being expended in that the heat transfer to the cryogen causes the cryogen to boil off into cryogenic vapor which is vented from the refrigeration system. Thus, the cryogen must be continually replenished. The hydrocooler reduces the operating costs involved in continually replenishing the cryogen because the initial heat transfer to the cooling water in the hydrocooler decreases the cryogen requirements in the cryogenic freezer by pre-chilling the articles of food. As may be appreciated, any refrigeration effect requires a predetermined amount of heat transfer. The distribution of such heat transfer between a hydrocooler and a cryogenic freezer decreases the amount of heat transfer between the cryogen and the food and thus, the cryogen requirement in the cryogenic freezer. It is appropriate to point out that the hydrocooler may also serve to rinse debris and surface contaminants from the articles to be eventually frozen.
In an additional conservation method, which can form still another cooling stage of a food processing system, cryogenic vapor, such as boiled off in a liquid nitrogen immersion freezer, may be used to pre-cool the articles of food prior to immersion in order to conserve the amount of cryogen needed for the particular refrigeration operation. A problem with using cryogenic vapor in this manner is that food articles tend to become dehydrated; and such dehydration results in a loss of food flavoring. Also, such pre-chillers using cryogenic gas are relatively inefficient at heat removal.
As may be appreciated, refrigeration systems, such as those discussed above, are run for long periods of time. One major expense involved in the long running period of such refrigeration systems is in the rate of cryogen usage. An additional expense involves water usage. As will be discussed hereinafter, the present invention conserves both the cryogen and water to reduce such expenses.